It could be an inflammation of the cartiledge of the sternum, costal condritis, or it could be a bit more complicated.
I found this and thought you should be aware of the possible injuries.
The sternoclavicular joint connects your collar bone to your sternum.
This joint does not have as much cartiledge holding it to your sternum which makes it unstable.
Injury to this area (car accidents, sports injuries, falling down, weight training etc) may cause this joint to dislocate and will cause pain in your chest and shoulder area.
The joint can be dislocated in two ways.
There is an anterior dislocation which means your clavicle is pushed in front of your sternum.
There is also a Posterior dislocation which means it is pushed to behind your sternum. Posterior is more dangerous to your vital organs.
When it is anterior you may feel a hard bump in your chest from the bone, and there will be heavy chest pain and possible shoulder pain as well.
You may have damaged the intra-articular disc ligament which would cause a grating, clicking, or popping feeling. I would bring this discussion up with your doctor and he will probably give you an x-ray to confirm the dislocation.
If it doesn't heal properly, you can end up with sternum-manubrial disruption, which may require surgery.